The triangle is supposed to rotate in response to touch events, but nothing happens. It also didn't rotate in the earlier step when it was supposed to rotate on its own. Not sure what the problem is. I just copied their code.

I believe this final multiplyMM is your problem. You are resetting your mMVPMatrix to be
mMVPMatrix = mProjMatrix * mVMatrix, and you are losing your model transformation matrix mMMatrix which would apply your rotation. Also, verify that mAngle is non-zero.